I have an Enterprise grade Cisco router and an HP laptop. I would like to connect to the router on the console port to conduct out-of-band management.

The console port on the router uses an RJ45 terminal. I have one on my laptop as well. My understanding is that I can not connect directly like that, but rather must use a DB9-RJ45 adapter.

This is fine, I just have to order one and wait. But I'd like to understand why that is and to ask the question "can't whatever change is being made via the hardware adapter be addresses by software?", just to improve my understanding.

  • 1
    Why do you need to order one? They all come with one. Did you toss it out? I have so many because all the Cisco equipment comes with one.
    – Ron Maupin
    Commented Oct 1, 2017 at 19:29
  • 1
    @Ron I bought it used
    – Hack-R
    Commented Oct 1, 2017 at 19:44
  • 4
    On your computer you have an Ethernet port with an RJ45 connector, and a RS-232 port with a DB9 connector, on the switch you have an RS-232 port with an RJ45 connector. USB may have spoiled us (especially with Macbooks now charging over USB) but you can't plug two completely different ports together and expect anything useful to happen, even if they happen to look the same. Commented Oct 1, 2017 at 22:33
  • You could easily build your own adapter (scroll down to "RJ-45 Port and Adapter Pin-outs"). You just need a RJ-45 connector/port and a DB9 connector and some wires (easily done by cutting your usual ethernet cable and doing some soldering onto a DB9 connector).
    – Batman
    Commented Oct 2, 2017 at 3:55
  • @Batman I appreciate the idea. I like to build stuff, but for now I'm short on time and supplies so I ordered one on eBay ;)
    – Hack-R
    Commented Oct 2, 2017 at 13:32

1 Answer 1


While using a classical RJ45 jack, the connection on your Cisco router really is a RS232 serial connections.

This means that, independently from the physical connector, your PC need to speek the same RS232 protocol. This is why do you need a DB9 (classic serial port connector) to RJ45 converter. The converter does not change the underlying protocol, rather it only provided the correct wirings for the right pins.

On the other hand, your PC's RJ45 connector is not internally connected to a serial port; rather, it is a direct extension of your network card, which speak a completely different protocol (even at the basic electrical layer).

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